The printing plate material for CTP, which is inexpensive, can be easily handled, and has a printing ability comparable with that of a PS plate, is required accompanied with the digitization of printing data. Recently, a thermal processless printing plate material which can be applied to a printing press employing a direct imaging (DI) process without development by a special developing agent or a versatile thermal processless printing plate material which can be treated in the same manner as in PS plates has been required.
The thermal processless type printing plate material is divided into an ablation type printing plate material, and a development-on-press type heat fusible image formation layer-containing printing plate material from the viewpoint of the image formation mechanism. The ablation type printing plate material has problems that it is low in sensitivity due to its image recording mechanism, and it is necessary to provide, in an exposure device, a sucking device for preventing a part of the image formation layer from scattering during laser exposure of the printing plate material.
The development-on-press type heat fusible image formation layer-containing printing plate material is more advantageous than the ablation type printing plate material, in that it is high in sensitivity, and it does not scatter a part of the image formation layer during laser exposure of the printing plate material.
However, it is necessary that a development-on-press type printing plate material such as the heat-fusible image formation layer-containing printing plate material above have, as an outermost layer or a layer under a water soluble layer protective layer, an image formation layer containing a water soluble component or a water swellable component, the image formation layer being such that when the layer is brought into contact with a dampening roller, an inking roller, or a blanket cylinder, the layer is likely to be removed at non-image portions. However, an aqueous solution or an organic solvent is likely to penetrate such a layer, and the above plate has problem in that the component contained in the layer is influenced by adherence of water or an organic solvent to the layer, resulting in image formation failure or development-on-press failure.
Herein, “a development-on-press type printing plate material” means a printing plate material comprising a layer capable of being removed with dampening water or printing ink on a printing press.
Examples of the heat-fusible image formation layer development-on-press type printing plate include a printing plate material comprising a hydrophilic layer or a surface-roughened aluminum plate and provided thereon, an image formation layer containing thermoplastic particles and a water-soluble binder (See for example, Japanese Patent O.P.I. Publication Nos. 9-123387 and 9-123387). However, even the plate having the structure also has problem in that adherence of water or an organic solvent to the image formation layer results in image formation failure or development-on-press failure.
A printing plate (or a printing plate material) is mounted on a plate cylinder of a press as follows. Being gripped at the leading edge of the printing plate with the plate leading edge gripper of the plate cylinder, the printing plate is nipped between the plate cylinder and a blanket cylinder, is rotated in such a direction that the plate is wound around the plate cylinder while pressing the plate to the plate cylinder by means of the blanket cylinder, whereby the plate is wound around the plate cylinder, and then is gripped at the trailing edge of the plate with the plate trailing edge gripper of the plate cylinder. Thus, the printing plate is wound around the plate cylinder and fixed on the plate cylinder. This method also applies in a DI press in which laser exposure is carried out.
However, when the development-on-press type printing plate material is employed, this method has problem in that water or an oleophilic substance-containing solvent, each remaining on a blanket cylinder, is transferred to the plate cylinder to the printing material, resulting in image formation failure or development-on-press failure.
The surface of the blanket cylinder is washed with a solvent or an aqueous solution every one round of printing. Accordingly, in order to obtain prints with stable quality employing the development-on-press type printing plate material, it is important to keep the blanket cylinder surface clean.
Hitherto, when the development-on-press type printing plate material is employed, any condition of the blanket cylinder surface has not been taken into consideration.